The present invention relates generally to an electronic keyboard musical instrument, and in particular to a percussion-type instrument such as an electronic piano or harpsichord having a velocity sensitive keyboard so that tones of higher amplitude will be produced the harder the keys are struck.
In a conventional acoustic piano, the strings are struck by hammers that are actuated by pressing keys of the keyboard so that the harder the key is pressed the greater is the force with which the hammers strike the strings thereby producing percussive tones of greater amplitude. A piano tone generally has a percussive envelope which will decay out with time regardless of whether the key is held and will snub off more quickly if the key is released. This type of percussive envelope is normally referred to as an ADSR envelope and has an attack portion of increasing amplitude, a rapid decay portion for a short time, a longer sustain portion of lower slope extending for a variable time depending on how long the key is held, and a release portion of high slope that occurs when the key is released and the damper contacts the strings thereby rapidly dampening out further vibrations.
A number of approaches have been taken in the past to develop an electronic instrument to simulate the sounds of an acoustic piano or harpsichord, but such instruments have generally been unsatisfactory. One widely used technique is to utilize analog circuits which are typically duplicated for each key of the keyboard. Such analog systems usually employ resistor-capacitor circuits that provide the expotenial decay characteristics of the decay and sustain portions of an envelope for a piano sound.
In order to detect the amount of force with which the key is struck, one commonly used technique is to time the movement of a keyswitch as it moves from one bus to another by using resistor-capacitor circuits or switched capacitor techniques. A further technique for detecting the velocity is to use piezoelectric devices that have alterable electronic characteristics depending on the amount of mechanical force with which they are struck.
One of the primary disadvantages to analog techniques used in the past is that the analog circuitry would potentially be duplicated for each key of the keyboard. This leads to an increase of the overall cost of the system, and if it is desired to change the attack and decay or velocity characteristics, the modification must be duplicated for each key of the keyboard. A further disadvantage is that the components making up the analog circuits are often not perfectly uniform so that unwanted differences in attack, decay and velocity characteristics may occur from key to key.
The concept of time sharing circuitry among a plurality of keys of the keyboard has been employed in the past in electronic musical instruments, such as electronic pianos and electronic organs. The circuitry in these instruments has not had the flexibility, controllability and economy of parts which are desirable in order to produce authentic piano-like sounds at a sufficiently low enough cost.